Realization that the fossil fuel supply of the United States is finite and may be rapidly depleted if the projected growth rate of national energy consumption continues has led to a search for substitute energy sources. Among a number of yet unexploited resources, those which have the least adverse environmental impact and are independent of foreign supplied deserve the most attention. Utilization of solar radiation is one of the possibilities for satisfying part of the nation's need for clean and reliable energy.
The direct conversion of sunlight into electricity by photovoltaic cells or its collection by heat collectors are proven techniques of solar energy utilization. They are now used in specialized applications where cost is not of importance. To be useful on a large scale, in competition with existing energy sources, the development of technology is needed to enable the economic production of efficient photovoltaic cells and solar heat collectors.
The combination of high optical transparency and high electrical conductivity exhibited by cadmium stannate (Cd.sub.2 SnO.sub.4) can be exploited for use as transparent electrodes in photovoltaic cells. This is fully discussed in a copending U.S. patent application, Ser. No. 181,916 and in a paper published in Physical Review, Vol. 6, No. 2, pp 453-59 dated July 15, 1972, entitled: "Optical and Electrical Properties of Cd.sub.2 SnO.sub.4 : A Defect Semiconductor," by A. J. Nozik.
Utilization of solar energy for space heating or air conditioning (absorption refrigeration) by collecting solar radiation in heat collectors has been suggested in a number of publications, including: Physics Today, Vol. 25, pp 44-49 of February 1972; Natural Resources J. Vol. 10, 268-326 (1970); Solar Energy, Vol. 13, pp 395-401 (1972); and, Technical Report AFML-TR-70-294, pp 21-44 of January 1971. In the referenced Physics Today article, it has also been proposed to use solar heat collectors in large-scale solar "farms" and to operate conventional central power stations with the collected heat.
Two types of solar heat collectors are under consideration, flat-plate collectors and pipe collectors. In flat-plate collectors, unconcentrated solar energy is absorbed by flat plates; pipe collectors absorb sunlight which has been concentrated by suitable lenses and mirrors. The light absorbing element in both devices is a coating with a large optical absorption coefficient, .alpha., over most of the solar spectrum. The collection efficiency depends on .alpha. and on heat losses, which consist of convection, conduction and radiation losses. Convection and conduction losses can be eliminated by placing the collecting surface in a vacuum. Radiation losses consist of infrared radiation and are temperature dependent, becoming more severe with increasing operating temperature of the collecting surface. These losses can be minimized by using selective absorber surfaces which have a large .alpha. for sunlight but a small emissivity, .epsilon., for heat radiation. Such surfaces are feasible, and in fact have been prepared since the solar spectrum and the heat radiation spectrum (up to at least 600.degree. C.) have little overlap -- See J. Opt. Soc. Am. 46, 31 (1956) and Technical Rept. AFML-TR-65-317 (October 1965 ).
Further reduction of radiation losses can be achieved by a greenhouse window coating deposited adjacent to the solar heat absorbing surface. The requirements for this coating are high transmittance for the incident solar radiation and high reflectance for heat radiation escaping from the absorber surface. These requirements are met by electrically conductive films of Cd.sub.2 SnO.sub.4 having a conductivity greater than 10.sup.3 ohm.sup..sup.-1 cm.sup..sup.-1. These films can be readily formed by the radio-frequency sputtering technique described in a copending U.S. patent application, Ser. No. 181,916, filed Sept. 20, 1971 or by chemical spraying methods wherein a solution of cadmium and tin bromides in the presence of H.sub.2 O.sub.2 and O.sub.2 is sprayed onto hot substrates at 400.degree. to 1000.degree. C.
The available heat flux from solar heat collectors, consisting of black body absorbers and greenhouse windows or of selective absorbers and greenhouse windows, is a function of solar concentration and absorber temperature. For high solar concentrations (&gt;10) and absorber temperatures below 500.degree. C., it has been shown in Technical Report AFML-TR-70-294 that a greenhouse window does not add to the available heat flux. However, for lower solar concentrations, the addition of a greenhouse window contributes significantly to the available heat flux. Obviously, these investigations show that the highest heat flux contribution of a greenhouse window coating can be realized in flat-plate collectors and in pipe collectors which work at high operating temperatures.
Unfortunately, the solar heat collectors as presently known do not provide for highly efficient heat collection. This lack of efficiency relates primarily to the re-radiating of substantial amounts of infrared thermal energy by the heat collecting surface.
It is an object of this invention to provide a solar heat collector of improved efficiency.
It is a further object of this invention to provide a solar heat collector having a greenhouse window which increases the heat flux available to the heat collecting system of a solar energy converter.
Other objects and attendant advantages of the present invention will be apparent from the description thereof taken in connection with the accompanying drawings. The invention is capable of a variety of mechanical expressions, two of which are illustrated in the accompanying drawings. Therefore, it is to be expressly understood that the drawings are for the purpose of illustration only, and are not intended to present the full scope of the invention which is defined by the appended claims.